Abstract

Developing high-performance Fe-based ammonia catalysts through simple and cost-efficient methods has received an increased level of attention. Herein, we report for the first time, the synthesis of two-dimensional (2D) FeOOH nanoflakes encapsulated by mesoporous SiO2 (mSiO2) via a simple solution-based method for ammonia synthesis. Due to the sticking of the mSiO2 coating layers and the limited spaces in between, the Fe after reduction retains the 2D morphology, showing high resistance against the sintering in the harsh Haber–Bosch process. Compared to supported Fe particles dispersed on mSiO2 spheres, the coated catalyst shows a significantly improved catalytic activity by 50% at 425 °C. Thermal desorption spectroscopy (TDS) reveals the existence of a higher density of reactive sites for N2 activation in the 2D Fe catalyst, which is possibly coupled to a larger density of surface defect sites (kinks, steps, point defects) that are generally considered as active centers in ammonia synthesis. Besides the structural impact of the coating on the 2D Fe, the electronic one is elucidated by partially substituting Si with Al in the coating, confirmed by 29Si and 27Al magic-angle spinning nuclear magnetic resonance (MAS NMR). An increased apparent activation energy (Ea) of the Al-containing catalyst evidences an influence on the nature of the active site. The herein-developed stable 2D Fe nanostructures can serve as an example of a 2D material applied in catalysis, offering the chance of a rational catalyst design based on a stepwise introduction of various promoters, in the coating and on the metal, maintaining the spatial control of the active centers.

Highlights

  • Catalytic conversion of N2 and H2 into NH3 through the Haber−Bosch process is one of the most important inventions of the 20th century

  • We report the fabrication of SiO2-encapsulated FeOOH nanosheets as a catalyst precursor for ammonia synthesis through a facile and low-cost solution-based method

  • Atomic-scale Transmission electron microscopy (TEM) characterization of the catalyst performed after in situ activation and a series of catalytic tests implies that the catalyst on work contains 2D Fe nanostructures with the presence of abundant surface step/kink sites

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Summary

INTRODUCTION

Catalytic conversion of N2 and H2 into NH3 through the Haber−Bosch process is one of the most important inventions of the 20th century. Earlier studies have evidenced that ammonia synthesis is a structure-sensitive reaction.[3,18−21] Surface sites including kinks, steps, and point defects are suggested as the active centers for dsyinsstohceisaist.i2o2n−24oTf hNer2e,fotrhe,e rate-limiting to gain a high step in activity in ammonia ammonia synthesis, developing catalysts that contain abundant surface sites is preferential. Another important concern lies in the stability of the active sites during the reaction. A special focus is given on the catalyst structure and morphology after testing in ammonia synthesis by high-resolution transmission electron microscopy (HRTEM) and three-dimensional (3D) tomography

MATERIALS AND METHODS
RESULTS AND DISCUSSION
CONCLUSIONS
■ ACKNOWLEDGMENTS
■ REFERENCES
Introduction

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